I believe the mangaka is pulling here The Twin Paradox thought experiment (from Einstein's General Relativity Theory) when it comes to space travel in near light speed (assuming that is the speed the characters that explore space are traveling).
Which means the ones who stay on Earth age faster relatively to the ones that travel through space away from Earth and back. Or to put this the other way, time goes slower for the ones that do a round trip travel near light speed.
As of why Motoka doesn't seem to age, there isn't much hints so far to guess. Jupiter's gravity isn't so great to slow down time that much on Motoka's moon, as the water planet orbiting the Black Hole did in the Interstellar movie.